from django.db import models
import uuid # Required for unique book instances
# Create your models here.
class Genre(models.Model):
    """
    Model representing a book genre (e.g. Science Fiction, Non Fiction).
    """
    name = models.CharField(max_length=200, help_text="Enter a book genre (e.g. Science Fiction, French Poetry etc.)")

    def __str__(self):
        """
        String for representing the Model object (in Admin site etc.)
        """
        return self.name
from django.urls import reverse #Used to generate URLs by reversing the URL patterns

class Book(models.Model):
    """
    Model representing a book (but not a specific copy of a book).
    """
    title = models.CharField(max_length=200)
    author = models.ForeignKey('Author', on_delete=models.SET_NULL, null=True)
    # Foreign Key used because book can only have one author, but authors can have multiple books
    # Author as a string rather than object because it hasn't been declared yet in the file.
    summary = models.TextField(max_length=1000, help_text="Enter a brief description of the book")
    isbn = models.CharField('ISBN',max_length=13, help_text='13 Character <a href="https://www.isbn-international.org/content/what-isbn">ISBN number</a>')
    genre = models.ManyToManyField(Genre, help_text="Select a genre for this book")
    # ManyToManyField used because genre can contain many books. Books can cover many genres.
    # Genre class has already been defined so we can specify the object above.

    def display_genre(self):
        """
        Creates a string for the Genre. This is required to display genre in Admin.
        """
        return ', '.join([ genre.name for genre in self.genre.all()[:3] ])
    display_genre.short_description = 'Genre'

    def __str__(self):
        """
        String for representing the Model object.
        """
        return self.title


    def get_absolute_url(self):
        """
        Returns the url to access a particular book instance.
        """
        return reverse('book-detail', args=[str(self.id)])

    import uuid
    # Required for unique book instances



class Author(models.Model):
    """
    表示作者的模型。
    """
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)
    date_of_birth = models.DateField(null=True, blank=True)
    date_of_death = models.DateField('去世日期', null=True, blank=True)

    def get_absolute_url(self):
        """
        返回访问特定作者实例的 URL
        """
        return reverse('author-detail', args=[str(self.id)])

    def __str__(self):
        """
        表示模型对象的字符串
        """
        return '%s, %s' % (self.last_name, self.first_name)

class BookInstance(models.Model):
        """
        Model representing a specific copy of a book (i.e. that can be borrowed from the library).
        """
        id = models.UUIDField(primary_key=True, default=uuid.uuid4,
                              help_text="Unique ID for this particular book across whole library")
        book = models.ForeignKey('Book', on_delete=models.SET_NULL, null=True)
        imprint = models.CharField(max_length=200)
        due_back = models.DateField(null=True, blank=True)

        LOAN_STATUS = (
            ('m', 'Maintenance'),
            ('o', 'On loan'),
            ('a', 'Available'),
            ('r', 'Reserved'),
        )

        status = models.CharField(max_length=1, choices=LOAN_STATUS, blank=True, default='m',
                                  help_text='Book availability')

        class Meta:
            ordering = ["due_back"]

        def __str__(self):
            """
            String for representing the Model object
            """
            return '%s (%s)' % (self.id, self.book.title)